Re: [PATCH 09/11] kexec_file: Remove mis-use of sh_offset field

[Philipp Rudo]

Hi Dave,


On Fri, 9 Mar 2018 11:14:20 +0800
Dave Young <dyoung@xxxxxxxxxx> wrote:

> On 02/26/18 at 04:16pm, Philipp Rudo wrote:
> > The current code uses the sh_offset field in purgatory_info->sechdrs to
> > store a pointer to the current load address of the section. Depending
> > whether the section will be loaded or not this is either a pointer into
> > purgatory_info->purgatory_buf or kexec_purgatory. This is not only a
> > violation of the ELF standard but also makes the code very hard to
> > understand as you cannot tell if the memory you are using is read-only or
> > not.
> > 
> > Remove this mis-use and store the offset of the section in
> > pugaroty_info->purgatory_buf in sh_offset.
> > 
> > Signed-off-by: Philipp Rudo <prudo@xxxxxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/kernel/machine_kexec_64.c | 10 ++++++----
> >  kernel/kexec_file.c                | 33 +++------------------------------
> >  2 files changed, 9 insertions(+), 34 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> > index 51667c8b5c9b..41db74bdc88b 100644
> > --- a/arch/x86/kernel/machine_kexec_64.c
> > +++ b/arch/x86/kernel/machine_kexec_64.c
> > @@ -457,13 +457,15 @@ int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
> >  		 * rel[i].r_offset contains byte offset from beginning
> >  		 * of section to the storage unit affected.
> >  		 *
> > -		 * This is location to update (->sh_offset). This is temporary
> > -		 * buffer where section is currently loaded. This will finally
> > -		 * be loaded to a different address later, pointed to by
> > +		 * This is location to update. This is temporary buffer
> > +		 * where section is currently loaded. This will finally be
> > +		 * loaded to a different address later, pointed to by
> >  		 * ->sh_addr. kexec takes care of moving it
> >  		 *  (kexec_load_segment()).
> >  		 */
> > -		location = (void *)(section->sh_offset + rel[i].r_offset);
> > +		location = pi->purgatory_buf;
> > +		location += section->sh_offset;
> > +		location += rel[i].r_offset;
> >  
> >  		/* Final address of the location */
> >  		address = section->sh_addr + rel[i].r_offset;
> > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > index 746b91e46e34..25b44d1a664a 100644
> > --- a/kernel/kexec_file.c
> > +++ b/kernel/kexec_file.c
> > @@ -736,28 +736,6 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
> >  	       pi->ehdr->e_shnum * sizeof(Elf_Shdr));
> >  	pi->sechdrs = sechdrs;
> >  
> > -	/*
> > -	 * We seem to have multiple copies of sections. First copy is which
> > -	 * is embedded in kernel in read only section. Some of these sections
> > -	 * will be copied to a temporary buffer and relocated. And these
> > -	 * sections will finally be copied to their final destination at
> > -	 * segment load time.  
> 
> It would be good to keep above part comment somewhere..

there is a comment in include/linux/kexec.h for the purgatory_info->sechdrs
field saying 

Temporary, modifiable buffer for sechdrs used for relocation.        
This memory can be freed post image load

When you think this is not enough i can add a comment here like

The section headers in kexec_purgatory are read-only. In order to have them
modifiable make a temporary copy.

What do you think.

Thanks
Philipp

> > -	 *
> > -	 * Use ->sh_offset to reflect section address in memory. It will
> > -	 * point to original read only copy if section is not allocatable.
> > -	 * Otherwise it will point to temporary copy which will be relocated.
> > -	 *
> > -	 * Use ->sh_addr to contain final address of the section where it
> > -	 * will go during execution time.
> > -	 */
> > -	for (i = 0; i < pi->ehdr->e_shnum; i++) {
> > -		if (sechdrs[i].sh_type == SHT_NOBITS)
> > -			continue;
> > -
> > -		sechdrs[i].sh_offset = (unsigned long)pi->ehdr +
> > -						sechdrs[i].sh_offset;
> > -	}
> > -
> >  	offset = 0;
> >  	bss_addr = kbuf->mem + kbuf->bufsz;
> >  	kbuf->image->start = pi->ehdr->e_entry;
> > @@ -786,17 +764,12 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
> >  			kbuf->image->start += kbuf->mem + offset;
> >  		}
> >  
> > -		src = (void *)sechdrs[i].sh_offset;
> > +		src = (void *)pi->ehdr + sechdrs[i].sh_offset;
> >  		dst = pi->purgatory_buf + offset;
> >  		memcpy(dst, src, sechdrs[i].sh_size);
> >  
> >  		sechdrs[i].sh_addr = kbuf->mem + offset;
> > -
> > -		/*
> > -		 * This section got copied to temporary buffer. Update
> > -		 * ->sh_offset accordingly.
> > -		 */
> > -		sechdrs[i].sh_offset = (unsigned long)dst;
> > +		sechdrs[i].sh_offset = offset;
> >  		offset += sechdrs[i].sh_size;
> >  	}
> >  
> > @@ -1006,7 +979,7 @@ int kexec_purgatory_get_set_symbol(struct kimage *image, const char *name,
> >  		return -EINVAL;
> >  	}
> >  
> > -	sym_buf = (char *)sec->sh_offset + sym->st_value;
> > +	sym_buf = (char *)pi->purgatory_buf + sec->sh_offset + sym->st_value;
> >  
> >  	if (get_value)
> >  		memcpy((void *)buf, sym_buf, size);
> > -- 
> > 2.13.5
> >   
> 
> Thanks
> Dave
>